1. Field of the Invention
The present invention relates to a hybrid vehicle employing a parallel hybrid system, using both an internal combustion engine and an electric motor/generator as a propelling power source, and specifically to a driving torque control system for a hybrid vehicle with a parallel hybrid system.
2. Description of the Prior Art
As is generally known, later model passenger cars often employ a variable valve timing/lift control system to improve engine power output, for example, at low- and mid-speed range, and to improve fuel economy, and to reduce exhaust emissions. Generally, the variable valve timing control system is used to control an intake valve closing point or an intake valve close timing (herebelow abbreviated to "IVC") depending on engine speed and/or engine load. In recent years, there have been proposed and developed various parallel hybrid vehicles propelled by an internal combustion engine and/or an electric motor. The internal combustion engine of such a hybrid vehicle often adopts the previously-noted variable valve timing control system for altering the intake valve timing. One such parallel hybrid vehicle has been disclosed in Japanese Patent Provisional Publication No. 5-50865. In this type of parallel hybrid vehicles as disclosed in the Japanese Patent Provisional Publication No.5-50865, the output shaft of the engine is connected through a clutch to a motor shaft of an electric motor, to transmit driving torque through the motor shaft via a differential to drive wheels. In order to reduce fuel consumption, the parallel hybrid system executes a so-called idling stop where the engine is stopped during the vehicle standstill. The hybrid vehicle normally moves off on electric power of the electric motor. After the engine is started up, the hybrid vehicle is propelled by driving torque generated by the engine with the clutch engaged.